A glitched one month trial with XBL Gold for an indefinite period of time. It's apparently quite rare and hasn't been patched in the last 3 years, so I doubt they're going to fix it anytime soon.
Just take a picture of the billing page. It should show when you made the account and say that you are still on the trial (with no records of any other membership billing). I would say that unless you are quitting xbox, it probably isn't worth selling the account because you won't get a good enough price.